The need for obtaining highly accurate measurements of three-dimensional objects in a short period of time arises in many industries and fields of endeavor, especially in the manufacturing sectors. One example can be found in the semiconductor processing industry wherein there is a need for accurate measurements of semiconductor wafer carriers.
Wafer carriers include wafer cassettes which can be used for 200 mm semiconductor wafers, and front opening universal pods (FOUPs) which can be used for 300 mm wafers. These wafer carriers hold a number of semiconductor wafers in a substantially rigid housing. Typically these carriers include a plurality of slots into which the wafers are inserted.
Different wafer carrier types may be used depending upon the nature of the processing a wafer is undergoing. For example, during chemical cleaning stages, a wafer carrier constructed of a chemical resistant plastic material is typically used. However such wafer carriers can be subjected to high temperatures as well as physical stresses during the course of wafer processing. As a result, carriers can become warped or deformed after repeated use. For example, such warpage or deformation may cause the carrier slots to vary in alignment which in turn may alter the spacing and alignment of the wafers themselves when placed in the slots. If so, the locations of the wafers inside the carriers can vary outside the tolerance range of automated wafer handling devices, such as wafer handling tools and wafer carrier handling tools, which are used to transfer wafers from one carrier to another or to another location in the manufacturing process, or to transfer wafer carriers to another location.
These automated transfer devices often require the wafers to be seated within the wafer carriers at specified positions within certain tolerances. If the wafers are seated outside these tolerances as a result of carrier distortion or any other reason, then the wafer handling device, which typically operates at high speeds, may mishandle the wafer. Mishandling of the wafers can result in scratches on the wafers or wafer breakage. Moreover, chips or breakage from one wafer can cause damage to other wafers in the carrier thus resulting in substantial financial loss relative to both the value of the wafers themselves as well as production delays.
In order to reduce or eliminate the use of wafer carriers which, for any reason, may fall outside specified tolerances, it is desirable to measure certain features of the carriers, such as warpage, with a high degree of accuracy. While the prior art has provided mechanisms to measure such features, such mechanisms generally are either relatively expensive and slow, on the one hand, or are limited in their measuring abilities, on the other hand.